This invention relates to extension rods used in percussive drilling in the mining and construction industries. An extension rod transmits impact energy from a percussive drill to a drill bit, the percussive drill remaining above the surface of the ground and the drill bit penetrating below surface to a depth roughly equivalent to the length of the extension rod. Greater depths may be drilled by connecting two or more extension rods together with coupling sleeves to form an extension rod string.
There are a variety of names given extension rods as used in percussive drilling, such as drill rods, extension drill steel, sectional drill steel and extension rod. For the sake of uniformity throughout the specification, the term "extension rod" will be employed.
It has been noted in percussive drilling that bending or flexural straining has an important effect on extension rod life and that impact energy which is diverted into flexural waves has the effect of reducing energy available for breaking rock. For example, it has been noted that such flexural strain can be as high as 70% of the normal longitudinal strain, and that it is largely responsible for extension rod breakage. William A. Hustrulid, A Study of Energy Transferred to Rock and Prediction of Drilling Rates in Percussive Drilling, University of Minnesota Master of Science Thesis (1965). The flexural waves also increase surface stress up to about 50% and produce considerable tensile stresses at certain points, which contribute to a reduction in fatigue life of the extension rod. H. C. Fischer, "Stress Pulses in Percussive Drilling," International Symposium on Mining Research, Vol. II (1961). It is, therefore, not surprising that these flexural waves further reduce extension rod life in addition to the normal tensile and compressive stresses in percussive drilling. Curt Dahlin, "Factors Influencing the Life of Drill Steel Equipment," International Symposium on Mining Research, Vol. 1 (1961).
The effect of flexural strains on the life of extension rods can be compensated for by improving the resistance to flexing or bending. However, such as improvement should not be obtained by increasing the cross-sectional area of the rod zone because this will increase both the weight and cost of the extension rod.
In rotary drilling operations the fatigue life of drill collars has been enhanced by providing greater flexibility in the zone adjacent to the threaded ends of the collar. See for example U.S. Pat. No. 3,730,286. However, rotary drill collars are not subjected to high velocity strain pulses superimposed on the rotary forces, as in percussive drilling, and flexural pulses or waves are virtually not present. Extension rods of the present invention are provided with greater rigidity through an increase in the moment of inertia, this being accomplished without increases to the cross-sectional area, as will be explained in more detail hereinafter.
As is well known in the art, rotary drilling consists of a tri-cone bit connected to a rotary drilling machine by a very long drill string. The drill string and the drilling machine comprise a large static weight which rests upon the tri-cone bit, the rotation of which under the great static weight creates a high loading which grinds or bursts the rock. In contrast, percussive drilling involves transmission of impact energy in the form of high velocity strain pulses, superimposed on the rotary forces, as discussed previously, and from this viewpoint the weight of the extension rod string is virtually irrelevant. In effect, what is being described is the difference between a static and a dynamic system. Percussive drilling with an out of hole (above ground surface) drill is efficient down to depths of about 60 ft.; beyond this point penetration is very slow due to energy losses in the extension rod string. This technique is used in quarrying, road construction, underground drilling, and pipeline construction and is fast, efficient, and highly mobile. Because of these differences in operating procedure between percussive drilling and rotary drilling, extension rods used in percussive drilling are too small to be used in rotary drilling operations. A further distinction between these two types of drilling operations lies in the removal of cuttings from the drilling hole; in percussive drilling cuttings are generally removed by air, whereas in rotary drilling a drilling mud is used to create a liquid flow which will lift the cuttings from great depths. Also, the types of threads used in percussive drilling and rotary drilling are completely different. In rotary drilling, a machine is needed to unthread the drill collars, whereas in percussive drilling no special machine is needed. In fact, the extension rods may be unthreaded by simply terminating the rotary forces and maintaining the impact forces to loosen the threaded connections.